Well, sieges are an important aspect of both games so I expect they'll be pretty well done in either of them. So far DoF seams more interesting because it has "original" siege weapons like plant based siege ladders for Elves that sprout out of the ground and grow upwards towards the ramparts (don't know if this has been changed) or living siege towers/battering rams in the form of Trees of War + it's an RTS with an economy and units building so battles/sieges don't come down to just a single engagement.
When that first fantasy Warhammer game (forgot the exact name) came out I thought it was going to be similar to Total War games and was disappointed with that linear RPG-esque campaign, and this isn't going to change in Battle March or so I've read.